Alerting: Change default for max_attempts to 3. (#97461)

Currently the default is 1, this means that by default users will see transient
query errors reflected as alert evaluation failures, when often an immediate
retry is sufficient to evaluate the rule successfully.

Enabling retries by default leads to a better experience out of the box.

pkg/setting/setting_unified_alerting.go

@@ -49,7 +49,7 @@ const (
 	evaluatorDefaultEvaluationTimeout       = 30 * time.Second
 	schedulerDefaultAdminConfigPollInterval = time.Minute
 	schedulerDefaultExecuteAlerts           = true
-	schedulerDefaultMaxAttempts             = 1
+	schedulerDefaultMaxAttempts             = 3
 	schedulerDefaultLegacyMinInterval       = 1
 	screenshotsDefaultCapture               = false
 	screenshotsDefaultCaptureTimeout        = 10 * time.Second